1. Field of the Invention
The invention relates to a projection objective for microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits and other microstructured components. The invention also relates to a method for the production of such a projection objective.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers on a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of structures, which is arranged on a reticle, is thereby imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching or deposition process so that the top layer becomes structured according to the pattern on the reticle. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied on the wafer.
One of the essential aims in the development of the projection exposure apparatus used for production is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, and this generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
The size of the structures which can be defined depends primarily on the resolution of the projection objective being used. Since the resolution of the projection objectives becomes better as the wavelengths of the projection light become shorter, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm and 157 nm.
Another way of increasing the resolution is based on the idea of increasing the numerical aperture of the projection objective with the aid of an immersion liquid. To this end, the immersion liquid is introduced into the intermediate space which remains between the last lens on the image side of the projection objective and the photoresist, or another photosensitive layer to be exposed. Projection objectives which are specially designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.3 or 1.4.
The last lens on the image side of high-aperture immersion objectives is usually curved very convexly on the object side and planar on the image side, in order to keep the possible ray angles of incidence less than 90° and therefore prevent undesired total reflection. Since this lens is generally very thick, it is usually made of fluorspar (CaF2) or another cubic crystalline material, for example BaF2, LiF2 or mixed crystals such as Ca1-xBaxF2. In contrast to conventional lens materials, for instance synthetic quartz glass, these crystals are still sufficiently transparent even for DUV projection light.
As it has now been found, however, these crystals are intrinsically birefringent at wavelengths in the deep ultraviolet spectral range. The term optically birefringent refers to materials with an anisotropic refractive index. This means that for a light ray passing through the material, the refractive index depends on its polarization and its orientation with respect to the material. The term birefringence in the stricter sense refers to the maximum possible refractive index difference Δn of a birefringent material. Owing to the polarization-dependent refractive indices, an unpolarized light beam is generally split into two beam components with mutually orthogonal linear polarizations when it enters a birefringent material.
If birefringence occurs in a projection objective, this will lead, unless suitable countermeasures are taken, to intolerable contrast losses in the image plane where the photosensitive layer is arranged.
In order to reduce the intrinsic birefringence in CaF2 and similar cubic crystals as much as possible, it has been proposed to select the orientations of the crystal axes of a plurality of crystals in order to obtain at least approximately axisymmetric direction distributions of the birefringence, or even so that the birefringent effects of the individual optical elements substantially cancel out one another. In general, the crystal lattices are mutually rotated about one of the crystal axes.
In a lens which is made of a single birefringent crystal, it is not possible to achieve compensation or symmetrization of the birefringent properties. For this reason, US 2004/0105170 A1 proposes the designed splitting of lenses into two lens components, which are contact bonded to each other after having been rotated. A method in which the lens preform is made from individual plates contact bonded to each other, which differ from each other with respect to the orientation of the crystal axes, is described as even more favorable. The lens preform consisting of two or more individual plates is then grinded and polished as a whole in a manner which is known as such.
This concept is modified in US 2003/0137733 A1 in so far as the individual plates of which the preform is made consist of crystals with a complementary birefringent character, for example calcium fluoride on the one hand and barium fluoride on the other hand. The splitting of two plane-parallel plates which are the last optical elements on the image side of the projection objective, respectively into two individual plates with mutually rotated orientations of the crystal axes, is furthermore described.
These known approaches to resolving the problem of birefringence, however, cannot readily be applied to the last lenses on the image side of projection objectives with particularly high numerical apertures, such as those which are possible in a design for immersed operation. The reason for this is that very large angles of incidence can occur at the planar interfaces between the crystals with different orientations of the crystal axes. At least rays with large aperture angles (i.e. rays which make a very large angle with the optical axis) could then be totally reflected at this interface. The high numerical aperture, which would be possible per se, is therefore reduced again.
US 2003/0234981 A1 discloses a projection objective comprising a first lens that is the penultimate curved optical element on the image side and has a concave surface on the image side. The first lens is made of a CaF2 crystal in a [110] crystal axis orientation. The projection objective furthermore has a second lens, which is the last curved optical element on the image side. It has a convex surface on the object side and is also made of a CaF2 crystal in a [110] crystal axis orientation, which is rotated, in relation to the crystal axis orientation of the first lens, by 120° about a symmetry axis of the first lenses.